1. Field of the Invention
The present invention relates to a light reflection preventing configuration such as a reflection light absorbing plate for preventing incident light from being diffused in reflection or for reducing the reflection light, in particular to a display panel having a reflection preventing configuration and a light modulation layer at which incident light is diffused to generate a diffusion light, and more particularly to a projection type display apparatus employing the display panel as a light valve, and to a display apparatus employing the display panel as an image pick-up monitor for a video camera (hereinafter referred to as "a view-finder").
2. Description of the Prior Art
In recent years, research and development of a liquid crystal display panel has been actively pursued because the liquid crystal display panel has many advantageous features such as being light in weight and thin as compared to a cathode ray tube (CRT). Also, the liquid crystal display panel is employed as a display portion of a pocket TV or a view-finder of a video camera. However, it has many problems, such that, for instance, it is difficult for use as a large scale screen.
Therefore, much interest and attention has recently been focused on a projection type display apparatus which is adapted to project and enlarge an image picture displayed on a small display panel through a projection lens or the like for displaying an enlarged image picture on a large scale screen. It is to be noted here that, for a projection type display apparatus and a view-finder now placed on the market, employed is a twist nematic (hereinafter, referred to as, "TN") display panel utilizing a light twisting characteristic of a liquid crystal.
The display panel using TN liquid crystal is required to linearly polarize incident light by using a polarizing plate (polarizer) disposed on the light incident side of the panel and is also required to dispose a polarizing plate (analyzer) on the outlet side of the panel for detecting the modulated light. Namely, it is necessary to arrange, in total, two polarizing plates on the front and rear sides of the TN display panel. That is, one is a polarizer for linearly polarizing incident light and the other is an analyzer for detecting the modulated light.
Assuming that the pixel opening ratio of the liquid crystal display panel is 100% and the light amount incident on the polarizer is 100%, the light amount outgoing from the polarizer is 40%, the transmittance of the display panel is 80% and the transmittance of the analyzer is 80%. Therefore, the transmittance as a whole becomes 0.4.times.0.8.times.0.8=about 25%, resulting in the effective utilization of only 25% of the incident light. As a result, the TN liquid crystal display panel can realize merely a low luminance image display.
Most of the light lost in the polarizing plates and the like is primarily absorbed by the polarizing plates and converted into heat while heating the polarizing plates. The heat converted from the light also heats the display panel through radiation heat. In the case of a projection type display apparatus, the incident light amount applied to the polarizer is of more than several tens of thousands luxes. Therefore, in the case of using a TN display panel as a light valve for a projection type display apparatus, the polarizing plates and the panel are heated to a high temperature, resulting in a remarkable deterioration of the performance in a short time period.
In addition, the TN display panel is required to be coated with an orientation film and then subjected to a rubbing process. The rubbing processing increases the number of processes, resulting in an increase of the manufacturing cost. Furthermore, the number of pixels of the display panel for use in the projection type display apparatus has recently been increased to a large capacity of more than three hundred thousand pixels, and the size of each pixel is in a trend toward miniaturization. The miniaturization of the pixel size results in the formation of a large number of surface irregularities caused by signal lines, thin film transistors (TFTs) 14, and the like per a unit area. It is obvious that rubbing process can not be done advantageously due to the surface irregularities.
In addition, the miniaturization of each pixel in size makes the area for forming the TFTs and signal lines in one pixel relatively large, resulting in a decrease of the pixel aperture ratio. As an example, in the case where three hundred fifty thousand pixels are formed in a display panel of a three-inch diagonal, the pixel aperture ratio is about 30%. In the case of a display panel formed with one million and five hundred thousand pixels, there is also estimated a value of less than 10% as the pixel aperture ratio. These decreases of the pixel aperture ratio not only decreases the luminance of the displayed image but also results in heating of the display panel by the light irradiated on a part other than an incident light aperture, which accelerates the deterioration in the performance of the TN display panel.
In the TN display panel, light modulation is effected by varying the orientation state of liquid crystals by applying a voltage to the pixel electrodes. Polarizing plates are disposed respectively on the incident and outgoing sides of the TN display panel, with the polarizing axes of the polarizer and analyzer intersecting at right angles with each other. Generally, the TN display panel is used in a mode (referred to as, "NW mode" hereinafter) wherein a black display can be effected in the voltage application state.
The displayed image on the display panel under the NW mode is excellent in color reproductibility, but it has a problem of leakage of light from the periphery of the pixels. This phenomenon takes place due to the fact that liquid crystal molecules are oriented not in the regular direction but in the opposite direction, which orientation state is called an opposite chilled domain. This takes place due to the fact that the rising direction of the liquid crystal molecule becomes partly opposite due to the electric field generated between the pixel electrode and the source signal line, and the like. In the portion where the rising direction of the liquid crystal molecules are opposite, light passes through the analyzer located on the light outgoing side of the panel despite the application of voltage. Namely, the leakage of light takes place. If the rising direction of liquid crystal molecule is regular, no leakage of light will take place.
As a method for preventing light leakage, there is available a method of increasing width of a black matrix (referred to as, "BM" hereinafter) to be formed on the confronting electrodes, but it also reduces the pixel aperture area of the matrix and thus reduces the luminance of a display image, and therefore, it can not be regarded as a useful method.
In the display panel using TN liquid crystals as described above, it is required to use polarizing plates. In addition, because light leakage tends to take place at the peripheral portion of the pixel, the BM must be made thick. Therefore, the light utilizing rate is poor and the display image is low in luminance. The light applied to the BM heats the display panel and consequently raises the panel temperature, deteriorating the life of the display panel.
Similarly, the projection type display apparatus employing a TN display panel as a light valve is also poor in the light utilizing rate, and the luminance of the projection image picture is low in the screen.
In view of this fact, there has been proposed a projection type display apparatus employing a polymer dispersed (referred to as, "PD" hereinafter) display panel instead of using polarizing plates. As an example, an apparatus disclosed in Japanese Patent Laid-Open Unexamined Publication No. 94225/1991 can be enumerated. In the PD display panel used as a light valve in the projection type display apparatus, light modulation is effected by scattering or transmitting the incident light.
The operation of the PD display panel is briefly described with reference to FIGS. 70A and 70B. As shown in FIGS. 70A and 70B, liquid crystals 691 are shaped in a water droplet (hereinafter, referred to as "water droplet shaped liquid crystal") which are dispersed in polymer 692. TFT (not shown) and the like are connected to a pixel electrode 101 and voltage is applied to the pixel electrode 101 by switching on-off the TFT. Thus, the modulation of light is effected by changing the orientation direction of the liquid crystals dispersed on the pixel electrode 101.
In the state where no voltage is applied as shown in FIG. 70A, the respective water droplet shaped liquid crystals 691 are oriented in irregular directions. In this state, a difference in the refraction index takes place between the polymer 692 and the water droplet shaped liquid crystal 691 and the incident light is scattered.
When voltage is applied to the pixel electrode 101 as shown in FIG. 70B, the orientation directions of liquid crystal modules are aligned with each other. Assuming that the refraction index of the liquid crystals 691 when they are oriented in the regular direction is in advance set equal to the refraction index of the polymer 692, the incident light is not scattered but transmitted through and emitted from an array substrate 12.
The conventional PD display panel has also a problem in that the light diffused or scattered in the liquid crystal layer 16 is reflected at the interface between the array substrate 12 and air, and reflected at the interface between the confronting substrate 11 and air. The scattered light is irregularly reflected on the interface between the liquid crystal layer 16 and substrates 11 and 12, thus resulting in deterioration in the contrast of a display image.
A method for preventing the deterioration of contrast in a display image due to the irregular reflection and diffusion of light is taught in the Japanese Patent Unexamined Laid Open No. 341269/1993. In this conventional method, a thick transparent substrate is attached to a display panel with a light absorbing film formed on the side face of the transparent substrate, thus preventing the irregular or scattered reflection.
However, in this conventional technical concept, it is required to increase the thickness of the transparent substrate. In particular, in a large scale display panel having a large display area, the thickness of the transparent substrate must be made very thick because the thickness of the transparent substrate is proportional to the diagonal length of the effective display area.
The increased thickness of the transparent substrate leads to an increase in weight of the display apparatus, and therefore it is inconvenient when carrying. In addition, in the case where a display panel with the transparent substrate attached thereto is used as a light valve in a projection type display apparatus, it is too large to be arranged within an optical block.
This is because the light valve is required to be arranged within a limited space between a dichroic mirror for color separation and a dichroic mirror for color synthesis, and the limited space limits the construction of a back focus of a projection lens. In order to increase the distance of the back focus of the projection lens, the number of lens pieces used for projection is generally increased, which leads directly to a cost increase. In addition, in order to obtain sufficient space for the transparent substrate, the optical block becomes large in size, which directly leads to a large sized system design and an increase in weight.
Although described in the Japanese Patent Unexamined Laid Open No. 341269/1993, one of the main causes of the deterioration in display contrast on the PD display panel is the occurrence of secondary diffused or scattered irregular reflection light. The secondary diffused or scattered light means the light which is diffused or scattered in a liquid crystal layer, and then reflected on the interface between the confronting substrate and air, which is then incident again to the liquid crystal layer and scattered or diffused (secondary diffusion) therein. The secondary diffused light reduces the display contrast to a great extent. As a result, the respective light emitted from the light modulation layer undesirably forms a light ring, generating a specific luminance distribution. In this conventional display apparatus, the array substrate is made thick or made thick with a thick light absorbing film attached to the array substrate and the light absorbing films are formed in the non-effective areas of the transparent substrate.